The region includes
much of the Alaska Range, which culminated at 20,300—foot
Mount McKinley; the northern slopes of the western Chugach Mountains;
and most of the Talkeetna Mountains These areas are separated by
poorly drained, lake-dotted lowlands the most extensive of which
extends northward from the head of Cook Inlet.
The oldest geologic units in the region are Paleozoic clastic
andcarbonate rocks, exposed mainly in the Alsaka Range. Mesozoi
volcanic and clastic rocks, in which considerable limestone is interbedded
locally, make up the bulk of the bedded rocks in the Chugach Mountains.
Recent investigations in a neighboring area (Plafker and MacNeil,
1966) indicate that some of the clastic and interbedded volcanic
rocks of the Chugach Mountains probably are early Tertiary in age.
Large granitic batholiths of Jurassic Cretaceous, and Tertiary age
invaded the older sedimentary and volcanic rocks in the Talkeetna
Mountains and Alaska Range (Dutro and Payne, 1957; Grantz and others,
1963; Reed and Elliott, 1970), and there are smaller plutons in
the Chugach Mountains and other parts of the region. Part of a discontinuous
bell of small ultramafic bodies of probable late Mesozoic age that
extends from the southern tip of the Kenai Peninsula nearly to the
Wrangell Mountains is in the Anchorage district.
Tertiary continental deposits underlie Cook Inlet and large areas
in the Susitna Lowland and Matanuska Valley. Tertiary and Quaternary
volcanic rocks have been found in the Matanuska Valley I west of
Anchorage, and in the southern Alaska Range, where Augustine
and some of the highest peaks are active volcanoes. Most of the
lowlands are underlain by thick glacial and alluvial deposits.
along the shore of Cook Inlet, in the lower Matanuska Valley, and
in a belt on both sides of the Susitna River below Curry, most
the region is underlain by permafrost. Ice completely covered the
Cook Inlet-Susitna River region during the Pleistocene, spreading
from the Alaska Range far out to sea Ice still covers the highest
parts of the mountains, and valley glaciers extend many miles
their source areas; one of them in the Kahiltna Glacier, is more
than 35 miles long.
Gold and silver have been recovered from lodes in many parts
of the Cook Inlet-Susitna region and a little copper has been
from deposits in the Redoubt and Valdez Creek districts. These
lodes and others have been investigated as possible source of
iron, chromite, molybdenum, copper, lead, and zink (Berg and Cobb,
1967, p. 16—37, figs. 5—9; Reed and Eberleir I 1972).
Reconnaissance studies in the southern Alaska Range in 1969
and Elliott, 1970) indicate that metallic sulfide minerals
are common in and near granitic plutons. Float samples collected
in the vicinity of the Mount Estelle pluton in the southwestern
part of the Yentna district contained as much as 60 parts per
(about 1.7 fine ounces per ton) gold associated with chaleopyrite,
arsenopyrite, and other sulfides. The only large-scale production
was from the Willow Creek area north of Palmer where, between 1909
and World War II, gold-bearing quartz veins in the southern
zone of the Talkeetna batholith were the source of about 404,425
fine ounces of gold, about 5 percent of Alaska’s total
lode gold output.
Placer gold was discovered in the Cook Inlet-Susitna region in the
late 19th century, and mining has been continuous since the early
1900’s. Total placer gold production from the region probably
has been about 250,000 fine ounces, or less than 40 percent of the
region’s lode-gold production. Production from the Valdez
Creek district cannot be confidently separated from that of the
neighboring Chistochina district of the Copper River region.
The total for the two is somewhat less than 200,000 fine ounces,
of which probably about 20 percent should be credited to the Valdez
Creek district. The combined placer production of the Willow Creek
and Yentna districts through 1960 was about 204,350 fine ounces;
an unknown, but small amount has been produced since that time.
As the main gold-producing part of the Anchorage district has historically
been considered a part of the Kenai Peninsula region, its output
cannot be stated accurately; probably it was not more than a few
thousand ounces. Scant data suggest that not more than 275 ounces
of gold was recovered in the Redoubt district, where a single stream
was worked for a few years.